In order to improve the ignition performance of a spark plug without providing a noble metal tip to a ground electrode of the spark plug, it has previously been proposed to form a protruding portion by press working on the ground electrode. Patent Document 1 discloses a technique to form a protruding portion on a ground electrode by a press working process called “forge pressing”. Non-Patent Document 1 discloses a technique to form a protruding portion on a ground electrode by another press working process called “extrusion pressing”.